I. Field of the Invention
The present invention relates generally to echo cancellers, and more specifically to a system and method for avoiding false convergence due to the presence of tone in time-domain echo cancellation.
II. Description of the Related Art
It is common in conventional land-based telephone systems to connect subscriber equipment to a central office using a two-wire line (often called the customer or subscriber loop). However, for equipment separated from the central office by distances greater than 35 miles, the two directions of transmission are separated onto physically separate wires. This is referred to as a four-wire line. Thus, when one of the parties to a call is located at a great distance from the central office (e.g., when a subscriber makes or receives a long-distance call), the central office must connect a two-wire line to a four-wire line. The device used to make this connection is called a hybrid. Thus, a typical long-distance telephone circuit can be described as two-wire in the subscriber loop to the local hybrid at the central office, four-wire over the long-distance network to the distant hybrid at the distant central office, and two-wire from the distant hybrid to the distant party.
One consequence of using hybrids to connect four-wire lines to two-wire lines is impedance mismatch. As a result of impedance mismatch at the hybrid, the speech of a speaker at one end, may be reflected off the hybrid at the other end (the distant hybrid). The reflection causes the speaker to hear an annoying echo of his own voice. Over relatively short distances, where the echo temporally coincides with the actual speech, the echo is not noticeable. However, over longer distances, the delay between the actual speech and the received echo is greater, resulting in a noticeable echo. To minimize the undesirable effects of such echoes, echo cancellers have been employed in various forms.
One form of echo canceller is described in U.S. Pat. No. 5,307,405, entitled "Network Echo Canceller" issued Apr. 26, 1994 and assigned to the assignee of the present invention. The `405 `patent describes a system in which the impulse response of the unknown echo channel is identified and a replica of the actual echo signal is generated using adaptive filtering techniques. The echo replica is subtracted from the signal heading toward the far-end speaker to cancel the actual echo signal.
Specifically, an adaptive filter at the central office receives a reference signal from the signal received from the speaker at the far end. The adaptive filter uses this reference signal to produce the echo replica which is essentially an estimate of the echo. This estimate is subtracted from the return signal that is heading to the far end, thus canceling the speaker's echo from this signal. The subtraction results in a residual error signal which is used by the adaptive filter to update its taps according to an adaptation algorithm such as the Least-Mean Square (LMS) method. In essence, the adaptive filter learns the frequency response of the unknown channel by observing the response to the frequencies sent out in the far-end signal. In other words, the adaptive filter uses the far-end speech as a reference and adapts its filter taps to accurately filter out the echo signal.
A state machine is provided to control the operation of the echo canceller and to determine when the adaptive filter should be updated. Typically, the adaptation step size of the filter is initially set large so that the filter converges quickly (i.e. so the filter adapts to the channel quickly). Then, once the filter has converged, the step size is made small so the filter remains converged on the channel.